How to Draw Dot and Cross Diagram of Carbon Dioxide

A carbon dioxide molecule has one carbon atom and two oxygen atoms. The atoms are joined by covalent bonds.

Carbon has proton number 6, so it has 6 electrons.

First shell: 2 electrons
Second shell: 4 electrons

Carbon needs 4 more electrons to reach an octet.

Each oxygen atom has proton number 8, so it has 8 electrons.

First shell: 2 electrons
Second shell: 6 electrons

Each oxygen needs 2 more electrons to reach an octet.

Note: We can find the number of electrons from The Periodic Table. The number at the top of each element is the atomic number, which tells us the number of protons. In an atom, the number of protons and electrons is the same. Therefore, this number also represents the number of electrons.

How the bonds form

Carbon shares 2 electrons with the first oxygen and 2 electrons with the second oxygen. Each oxygen shares 2 electrons with carbon.

4 electrons are shared between carbon and each oxygen.
4 shared electrons make 2 bonds.
Each carbon–oxygen bond is a double bond.
Each oxygen still has 4 valence electrons that are not shared.
The inner shells are not involved in bonding.

Dot and cross diagram

Write C in the centre and O on each side.
Show 4 valence electrons around carbon. Use crosses for carbon.
Show 6 valence electrons around each oxygen. Use dots for oxygen.
Overlap each oxygen with carbon so that 4 electrons sit in each shared region: 2 from carbon and 2 from that oxygen.

After drawing the diagram, it is good to check whether each atom has achieved a noble gas electronic configuration (octet rule):

The oxygen on the left has 8 outer electrons. ✓
Carbon has 8 outer electrons. ✓
The oxygen on the right has 8 outer electrons. ✓

Note for exams: If the question asks you to draw only the outermost electrons, you can omit the inner shells. Only show the valence electrons involved in bonding.

Structural formula

Converting to structural formula: From the dot and cross diagram, we can see that 4 electrons are shared between carbon and each oxygen atom. Since 4 electrons are shared, this means 2 bonds are formed.

O=C=O

Therefore, carbon dioxide contains double bonds between carbon and oxygen.